It is permissible to install a roof over the top of an existing roof. By doing so the current owner of the building can save some money on the roofing project. However, roofing over another roof does not provide the same quality of roof, or offer the same level of security that a roof system installed when a complete removal was performed. This is where people say, “Building code, and manufacturers allow it. It must be all right.”
Let’s look how the code officials, and manufacturers look at roof over jobs. When installing asphalt composite shingles most Building Codes allow for up to 2 layers of asphalt roofing on a structure. The code has nothing to do with shingle performance. It is written in regards to the structural integrity of building, and how much weight it can handle. Manufacturers also prohibit roofing over an old roof. However if you read the fine print of any roofing manufacturer’s warranty you will find verbiage in their warranty exclusions that say “We will not warrant product failure if roof is installed over a substrate that is not smooth, flat, and clean.” This means that if anything under the roof causes the roof on top to fail, there is no warranty.
Other concerns about roofing over the top of a roof system are: There is no way to do a thorough roof deck inspection when it is covered by old roofing. This could result in damage being concealed. It is much more difficult to flash a second layer. If the second layer leaks the water can ride the layer underneath and travel to a different area of the roof. This makes leak detection and correction very difficult. Shingles are made from Asphalt. The flexibility of Asphalt comes from the natural solvents within it. A second layer can be super-heated in summer months which can cause the shingle to buckle and curl prematurely. Removal costs in the future will be tremendous due to the extra labor, and escalating landfill rates.
We believe that roofs should be removed so the property can be totally inspected, and any damage can be repaired before a new roof system is applied. If you are considering re-roofing, and are not sure of what your options are contact us for your free inspection.